1. Field of the Invention
This invention relates to semiconductor wafer processing reactors and, more particularly, to an improved clamp ring for clamping a semiconductor wafer onto a domed heater pedestal used in such processing reactors.
2. Brief Description of the Prior Art
Semiconductor wafers are generally processed in thermal reactors in which the wafer is subjected to a number of different processing steps. In certain wafer processes, for example physical vapor deposition (PVD), often called sputtering, the wafer to be processed is held down onto a domed heating pedestal by means of some sort of clamping device such as a clamp ring. More details of the PVD/sputtering process itself, and the apparatus used for such processes are disclosed in U.S. Pat. No. 5,108,569 (Gilboa et al), and are incorporated by reference herein.
An example of a typical PVD wafer processing reactor and how a wafer is mounted therein before processing commences is illustrated with reference to FIG. 1, which is an exploded sectional view of the relevant internal components of the reactor.
Before wafer processing commences, a semiconductor wafer 10 is laterally brought into the reactor by means of a robot arm (not shown). Thereafter, a horseshoe-shaped wafer support 12 moves up from below the wafer 10 until the wafer is supported clear of the robot arm by the flat support faces 14 of four lift fingers 16 (of which only two are shown). In this position the robot arm lies within the open part 17 of the horseshoe-shaped support 12. Thereafter, the robot arm leaves the reactor, and the horseshoe-shaped support 12 continues to move upward. At the same time a wafer heating pedestal 18 moves vertically up into position near the underside of the wafer 10. As is shown in this figure, both the horseshoe-shaped support 12 and the pedestal 18 move vertically up (or down at the end of the processing cycle) along a common, central axis 19, indicated in broken lines.
The pedestal 18 has a domed upper surface and has an outer diameter only slightly smaller than that of the outer diameter of the wafer 10. To enable the pedestal 18 to move up through the center of the support 12 and past the lift fingers 16, four cutouts 20 are formed in the sides of the pedestal. These cutouts are large enough to accommodate the fingers 16 when the pedestal 18 moves up close to the underside of the wafer 10.
The pedestal 18 is typically made of stainless steel and, although not shown, includes a heater element for heating the pedestal 18. This, in turn, heats the wafer 10 during processing operations. To ensure that transfer of heat from the pedestal to the wafer is uniform, gas is injected into the space between the wafer and the pedestal via a gas conduit 22 which is formed in the pedestal 18 and exits in the center of its domed surface. Further details of how this is achieved will be given below.
The horseshoe-shaped support 12 lifts the wafer until it is close to, but not in contact with, a generally circular clamp ring 24. Thereafter, the pedestal 18 pushes up against the underside of the wafer 10, lifting it off the flat faces 14 of the lift fingers 16, and bringing it into contact with a flat faced, annular seat 26 formed in the clamp ring 24.
The pedestal 18 continues to move upward until the clamp ring 24 is lifted clear of its supports (not shown) and is supported only by the wafer 10 resting on the pedestal 18. As the clamp ring 24 is fairly heavy (approximately 3 pounds or 1.35 kilograms), it forces the wafer 10 to adopt approximately the profile of the dome of the pedestal 18. This configuration is illustrated in greater detail in FIG. 2, which is an enlarged detail of the area where the clamp ring 24 engages the edge of the wafer 10. It is in this position that the PVD/sputtering process steps occur.
FIG. 2 illustrates clearly the problem with the prior art clamp rings. As is apparent from this figure, the seat 26 is horizontal, while the pedestal, and accordingly the wafer when it conforms to the shape of the pedestal, are curved. As a result, the point of contact between the heavy clamp ring 24 and the wafer 10 is a sharp right-angled edge 30. The problem with this arrangement is that the wafer's surface can be damaged by the edge 30 gouging into it. Such damage can lead to undesirable particle generation during the processing operations and can result in strain lines from which chipping or flaking of the layers (deposited during subsequent wafer processing) on the surface of the wafer 10 can occur.
This problem is further enhanced when gas is injected into the gas conduit 22. This gas is used to ensure uniform heating of the wafer and is forced into the space between the backside of the wafer 10 and the pedestal 18. The gas, usually an inert gas such as argon, in the space behind the wafer is typically at a pressure of between 4 to 12 Torr while the interior of the reactor is at 4 to 10 milliTorr. As a result of this pressure differential the wafer flexes and bows away from the pedestal 18, with the greatest separation between wafer and pedestal occurring at the center of the wafer, a position indicated by broken lines 10'. Not only does this flexing cause even greater pressure to be exerted on the wafer's surface by edge 30 of the clamp ring 24, but it also causes the edge 30 to scrape the surface of the wafer 10 as the flexing of the wafer causes its outer edge to move inward toward its center.
In the past, attempts have been made to overcome this problem by very accurately machining and precision polishing the seat 26 and particularly the right-angled edge 30. This machining and polishing process requires a great deal of care by skilled operators and is therefore very costly.
Unfortunately, even the most careful polishing and resulting demurring of the right-angled edge 30 is still not sufficient to provide a totally defect-free edge 30. Any imperfections in this edge 30 act as concentration points for the stress generated by the clamp ring as it holds the wafer down onto the pedestal. These stress concentration points cause damage which can be particularly troublesome as a silicon wafer behaves much like a piece of glass; i.e., a small chip or score mark on the wafer's surface may propagate from the point of stress and shatter the wafer. Additionally, such scoring and marking of the wafer's surface disrupts its planarity. Thus, subsequent processing, for example with a material such as tungsten-CVD, which is difficult to bond to a wafer's surface even under ideal conditions, may not be possible with any measure of reliability as the tungsten may not be able to adhere to the damaged portion of the wafer's surface. As a result, the tungsten may tend to lift away from the entire wafer's surface during further processing thereof.
Other problems with the components of the reactor include an unstable wafer support due to the open-ended horseshoe-shaped support 12 and inexact positioning of the fingers 16. This second problem results from the method of manufacture of the support 12--it is first machined and thereafter the fingers 16 are welded onto it. Unfortunately this welding is very difficult to achieve accurately and has resulted in misaligned fingers.
The need therefore exists for a better and improved manner of clamping a semiconductor wafer onto a domed pedestal in a thermal reactor as well as improving the design of some of the other components within the reactor.